Steels of the general compositions set out in U.S. Pat. Nos. 1,464,174 and 2,331,900 have been developed to a high degree of proficiency, and today such steels have acquired a reputation in the closed die forging industry for quality and durability; indeed, one, which is sold under the trademark FX, has long been recognized as a leading steel in this industry.
However, from the producers point of view there is still room for improvement because the problem of ingot panel cracking is occasionally encountered, and, from the users point of view, increased die life is always desirable.
Panel cracking is a condition which has long plagued the alloy steel maker. Panel cracks generally occur near the base of the ingot and extend in depth up to or beyond the mid-radial portion of an ingot. In ingots of up to about 9 inches thickness, panel cracking is quite rare, but the frequency of occurence increases with increasing ingot size over about nine inches. The cracks may be quite deep, and, when discovered, normal processing must be interrupted and the cracks ground or torched off, if possible, before processing can resume. Such rectification steps obviously increase the cost of production to a producer, if indeed the crack is sufficiently minor that the ingot can be salvaged for further processing.
It is well documented that the occurrence of panel cracking increases with greater frequency and severity in steels containing aluminum, and, further, that steels containing aluminum and nitrogen are particularly susceptible, with the severity of cracking increasing with an increase in nitrogen. For a fuller discussion of the problem, and general background information, reference is made to the Journal of the Iron and Steel Institute, March, 1959, pages 250-256. A further literature reference which speaks in terms of catastrophic deterioration in hot workability when aluminum nitride is present in steels at forging temperature is found in the Journal of the Iron and Steel Institute, January, 1964, pages 32-41.
As is well-known, aluminum is an essential element in closed die forging steels for a number of reasons, including its deoxidizing capability and its ability to ensure the attainment of fine grain size which is essential to long die life. However, as is well known to those skilled in the art, too much aluminum tends to coarsen grain size. Nitrogen is also inherently present in closed die forging steels due to the balance which must be struck between cost and quality in the steel making process.
Hence, the production of the type of steels here under discussion, and particularly the producer of closed die forging steels, must cope with the adverse characteristics of aluminum and nitrogen.
A highly desirable property in any of the type of steels under discussion, including closed die forging steels, is high durability, as measured by reduction of area transverse, in connection with all the other characteristics of such steels, including high strength, high hardness and wear resistance, and high corrosion and fatique resistence. Since such steels are forged, it is well known, up to now, that the reduction of area will always be greater in the longitudinal direction as contrasted to the transverse direction of the direction of forging. It is also well recognized that the ideal closed die forging steel should have the same high reduction of area in both a transverse and longitudinal direction so that the steel, when made into a die, will be equally resistant to cracking in all directions. Attainment of this goal would also greatly simplify die design, since the die designer can ignore the "grain" of the steel in designing and fabricating the die, and greater die life should be achievable.
Accordingly, a primary object of the invention is to provide a low alloy steel composition having a medium carbon content which is capable of use as a forging die in closed die forging and in which surface problems, such as panel cracking, in the manufacturing process have been eliminated, or the frequency thereof very greatly reduced as contrasted to the frequency of panel cracking currently encountered by the steelmaker.
Another object of the invention is to provide a low alloy steel composition having a medium carbon content which is capable of use as a forging die in closed die forging, and in which the reduction of area transverse is in the range of from about 15 percent to about 30 percent.
Yet a further object of the invention is to provide a low alloy steel composition having a medium carbon content in which the ratio of the reduction of area transverse to the reduction of area longitudinal closely approaches 1.0.
Yet a further object of the invention is to provide an efficient, reproducible, and economical method of producing steel having the properties described above.
Other objects and advantages of the invention will be apparent from the following description of the invention.